The mononuclear and dinuclear dimethoxyethane adducts of lanthanide trichlorides [LnCl3(DME)2]n, n=1 or 2, fundamental starting materials in lanthanide chemistry: Preparation and structures

Article abstract of DOI:10.1016/j.ica.2003.11.011

Some new dimethoxyethane (DME) adducts of lanthanide trichlorides of formula [LnCl₃(DME)₂]_n, n=1 or 2; (n=2, Ln=La, Ce, Pr, Nd; n=1, Ln=Eu, Tb, Ho, Tm, Lu) have been prepared by treating Ln ₂O₃, or LnCl₃·nH₂O, or Ln₂(CO₃)₃, in DME as medium, with thionyl chloride at room temperature, eventually in the presence of water in the case of Ln₂O₃ and Ln₂(CO₃)₃. The complexes from lanthanum to praseodymium included are chloro-bridged dimers. In the case of neodymium, the new results complement the literature data, showing that both the mononuclear and dinuclear species exist: neodymium can therefore be regarded as the turning element from dinuclear to mononuclear structures along the series. Only mononuclear complexes were isolated in the Eu-Lu sequence. The lanthanide contraction has been evaluated on the basis of the Ln-O and Ln-Cl bond distances on the isotypical series of the mononuclear complexes LnCl₃(DME)₂ covering a range of 12 atomic numbers.

Full text of DOI:10.1016/j.ica.2003.11.011

Inorganica Chimica Acta 357 (2004) 1538–1548
www.elsevier.com/locate/ica
The mononuclear and dinuclear dimethoxyethane adducts
of lanthanide trichlorides [LnCl3(DME)2]n, n ¼ 1 or 2,
fundamental starting materials in lanthanide chemistry:
preparation and structures
1
Ulrich Baisch , Daniela Belli DellÔAmico, Fausto Calderazzo, Roberto Conti,
2
*
Luca Labella, Fabio Marchetti , Elsje Alessandra Quadrelli
Dipartimento di Chimica e Chimica Industriale, Universitꢀa di Pisa, via del Risorgimento 35, I-56126 Pisa, Italy
Received 4 November 2003; accepted 8 November 2003
Abstract
Some new dimethoxyethane (DME) adducts of lanthanide trichlorides of formula [LnCl3(DME)2]n, n ¼ 1 or 2; (n ¼ 2, Ln ¼ La,
Ce, Pr, Nd; n ¼ 1, Ln ¼ Eu, Tb, Ho, Tm, Lu) have been prepared by treating Ln2O3, or LnCl3 ꢀ nH2O, or Ln2(CO3)3, in DME as
medium, with thionyl chloride at room temperature, eventually in the presence of water in the case of Ln2O3 and Ln2(CO3)3. The
complexes from lanthanum to praseodymium included are chloro-bridged dimers. In the case of neodymium, the new results
complement the literature data, showing that both the mononuclear and dinuclear species exist: neodymium can therefore be re-
garded as the turning element from dinuclear to mononuclear structures along the series. Only mononuclear complexes were isolated
in the Eu–Lu sequence. The lanthanide contraction has been evaluated on the basis of the Ln–O and Ln–Cl bond distances on the
isotypical series of the mononuclear complexes LnCl3(DME)2 covering a range of 12 atomic numbers.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Lanthanide complexes; Chloride complexes
1. Introduction
used in luminescence [3], in NMR spectrometry and in
organic synthesis [4]. When operating in non-aqueous
systems, the anhydrous trichlorides LnCl3 are the most
frequently used starting materials for the preparation of
many useful derivatives. Our method for preparing the
anhydrous trichlorides was reported in an earlier pre-
liminary communication [5] to give substantially quan-
titative yields of the dimethoxyethane (DME) adducts of
neodymium, europium, and gadolinium. It consists of
treating the readily available oxide with the three-com-
ponent system SOCl2/DME/H2O at room temperature,
whereby the addition of the appropriate, in principle
catalytic, amount of water generates HCl in situ, which
is used to convert the oxide, a primary product of the
lanthanide technology, into the chloride. The higher
coordinating power of DME with respect to SOCl2 ex-
cludes the formation of the adducts with thionyl chlo-
ride. Later methods using the metal oxide as starting
material employ HCl generated in situ from chlorotri-
Lanthanides continue to attract interest in both sci-
entific and technological fields, leading to traditional
and new applications. Thus, the industrial applications
to ceramic enamels, magnetic materials, and optical
glasses have recently been extended, for instance, to Er-
doped optical fibers [1] or to lanthanide-based catalysis
for diolefin polymerization [2]. Lanthanides are also
* Corresponding author. Tel.: +390502919233; fax: +390502919246.
E-mail address: Fama@dcci.unipi.it (F. Marchetti).
1
Present address: Department Chemie der Ludwig-Maximilians-
€
Universitat Munchen, Butenandtstrasse 5-13 (Haus D), D-81377
Munchen, Germany.
€
€
2
ꢁ
Present address: Laboratoire de Chimie Organometallique de
Surface UMR9986 – CNRS CPE, Boulevard du 11 Novembre 1918,
43, BP 2077, F-69616 Villeurbanne, France.
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.11.011

U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
1539
methylsilane and water [6]. Otherwise, the appropriate
bulk metal is sonicated with hexachloroethane in the
ether as medium [7]. The new preparative methods (vide
infra) [5–7] to anhydrous lanthanide chlorides appear to
be more facile than those based on the traditional high-
temperature (200 °C or higher) reaction of the lantha-
nide oxide with ammonium chloride [8a], or on the
treatment of the lanthanide metal with a chlorinating
agent (HgCl2) [8b].
The present paper, beside extending the SOCl2/DME/
H2O methodology [5] to La2O3, thus covering a range of
8 atomic numbers along the lanthanide series (from
lanthanum to gadolinium included), reports that anhy-
drous lanthanide chlorides can also be prepared from
the carbonate [the example of Er2(CO3)3(H2O)2 is ex-
plicitly presented in Section 2 or from the hydrated
chloride, simply by treating the starting material with
thionyl chloride at room temperature. As the hydrated
chlorides can be easily obtained from the oxide and
aqueous hydrochloric acid, this paper discloses three
different methods of preparing the DME adducts of the
anhydrous lanthanide chlorides, all operating at room
temperature and with easily available reagents.
At the onset of this work, lanthanide trichlorides in
the presence of an oxygen donor were known to form
ionic or low-nuclearity species, for instance Ln2Cl6-
(THF)7, i.e. [LnCl2(THF)5]þ[LnCl4(THF)2]ꢁ, or LnCl3-
(DME)2, LnCl3(THF)4, or the chloride-bridged dimer
Yb2Cl6(THF)4 [9]. In our preliminary communication
[5] we had already noted that the europium(III) deriv-
ative of formula EuCl3(DME)2 is isostructural with
both the gadolinium [10] and yttrium analogs [11]. At
the present time, structural characterization of some
mononuclear LnCl3(DME)2 species has been achieved
(Ln ¼ Nd [12], Dy [9a], Er [7b,9b], Yb [7b]) showing
these compounds to be isotypical with each other and
with the yttrium derivative YCl3(DME)2 [11].
Moreover, structural data have recently been re-
ported by an Australian group on the dinuclear tri-
chloro-dimethoxyethane derivative of neodymium of
formula Nd2Cl6(DME)4. The Authors have noted that
this chloride-bridged dimer contains octacoordinated
neodymium and they have indicated that ‘‘it remains to
be determined where the transition point from eight co-
ordinate to seven coordinate occurs and the structure of
SmCl3(dme)2 is needed to resolve this.’’ [9c]. As it was
somewhat intriguing that neodymium could give both
the monononuclear [12] and the dinuclear derivatives, it
was in order to clarify and rationalize several structural
aspects of this important class of lanthanide derivatives.
The present paper reports new crystal data on the
mononuclear species in the sequence from europium to
lutetium, see Table 1. Furthermore, it still remained to
be established which are the typical crystal structures for
the elements from lanthanum to praseodymium, i.e. in
the early part of the lanthanide sequence. Our paper
reports the X-ray structures of the dinuclear lantha-
num(III), cerium(III) and praseodymium(III) deriva-
tives, see Table 2. Thus, neodymium, for which both the
mononuclear and the dinuclear structures are known,
appears to be the turning point from dinuclear (CN
eight for the lanthanide) to mononuclear (CN seven for
the central metal atom) structures.
A further point of interest is represented by the
observation that for isostructural series of compounds,
the experimentally observed interatomic parameters
can be utilized for an evaluation of the ionic radii
along the lanthanide series [13]. It is to be pointed out
that the so-called lanthanide contraction is based on
crystal data collected around the fifties of the past
Table 1
Crystal data and structure refinement of the isotypical, monoclinic, space group P21=c, lanthanide complexes LnCl3(DME)2
Lanthanide
Eu
Tb
Ho
Tm
Lu
Empirical formula
Formula weight
Temperature (K)
C8H20Cl3EuO4
438.55
208(2)
C8H20Cl3O4Tb
445.51
293(2)
C8H20Cl3HoO4
451.52
293(2)
C8H20Cl3O4Tm
455.52
293(2)
C8H20Cl3LuO4
461.56
293(2)
ꢂ
a (A)
11.506(1)
8.9044(7)
15.644(2)
104.92(1)
1548.8(3)
4
11.501(2)
8.941(1)
15.670(2)
105.02(1)
1556.2(4)
4
11.456(1)
8.9272(8)
15.660(1)
104.962(7)
1547.3(2)
4
11.4166(9)
8.9161(7)
15.633(1)
104.919(7)
1537.7(2)
4
11.371(2)
8.889(2)
15.606(4)
104.825(15)
1524.9(6)
4
ꢂ
b (A)
ꢂ
c (A)
b (°)
Volume (A )
3
ꢂ
Z
qcalc (Mg/m3)
1.881
4.563
1.902
5.056
1.938
5.628
1.968
6.287
2.010
6.995
l (mmꢁ1
Data/restraints/parameters
)
3715/0/145
0.0311
0.0708
2746/0/146
0.0323
0.0788
3785/0/150
0.0350
0.0939
3007/0/145
0.0475
0.1061
2692/0/146
0.0543
0.1351
a
RðFoÞ½I > 2rðIÞꢂ
Rw (Fo2Þ½I > 2rðIÞꢂ
a
Goodness-of-fit on F 2a
1.019
h
1.039
0.830
1.019
1.012
.
i
1=2
P
P
P
P
2
2
2
a RðFoÞ ¼ jjFoj ꢁ jFcjj= jFoj; Rw(Fo2Þ ¼
½wðFo2 ꢁ Fc2Þ ꢂ
½wFo2Þ ꢂ
;
w ¼ 1=½r2ðFo2Þ þ ðAQÞ þ BQꢂ, where Q ¼ ½maxðFo2; 0Þ þ 2Fc2ꢂ=3;
h
i
1=2
P
2
Goodness-of-fit ¼
½wðFo2 ꢁ Fc2Þ ꢂ=ðN ꢁ PÞ
, where N and P are the numbers of observations and parameters, respectively.

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U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
Table 2
Crystal data at 293 K and structure refinement of Ln2Cl6(DME)4 complexes
Lanthanide
Laa
Cea
Pra
Nda
Ndb
Prc
Empirical formula
Formula weight
Space group
C16H40Cl6La2O8 C16H40Cl6Ce2O8 C16H40Cl6O8Pr2 C16H40Cl6Nd2O8 C20H50Cl6Nd2O10 C16H40Cl6O8Pr2
851.00
853.42
P21=n
855.00
P21=n
861.66
P21=n
951.78
ꢃ
855.00
P21=n
P21=n
P1
ꢂ
a (A)
10.947(3)
12.992(3)
11.225(7)
10.929(1)
12.983(1)
11.200(1)
10.907(2)
12.956(2)
11.163(1)
10.898(1)
12.935(1)
11.132(1)
8.645(2)
9.322(1)
12.676(2)
88.597(6)
83.88(1)
65.78(1)
926.1(3)
1
8.474(3)
18.149(5)
10.569(2)
ꢂ
b (A)
ꢂ
c (A)
a (°)
b (°)
c (°)
102.51(5)
102.31(1)
102.24(2)
102.16(1)
109.16(3)
3
ꢂ
Volume (A )
1558.6(11)
2
1.813
1552.6(2)
2
1.825
1541.6(4)
2
1.842
1534.0(2)
2
1.865
1535.4(7)
2
1.849
Z
qcalc (Mg/m3)
1.707
3.245
l (mmꢁ1
Data/restraints/parameters
)
3.251
3.444
3.676
3.903
3.691
2437/0/149
0.0579
0.1391
2.097
2607/0/149
0.0289
0.0670
0.998
2590/0/149
0.0239
0.0674
1.207
2303/0/149
0.0308
0.0691
1.005
2520/0/177
0.0287
0.0728
1.084
2533/0/149
0.0224
0.0594
1.154
d
RðFoÞ½I > 2rðIÞꢂ
Rw(Fo2Þ½I > 2rðIÞꢂ
d
Goodness-of-fit on F 2d
a a-Phase.
b Nd2Cl6(DME)4 ꢀ DME.
c b-Phase.
d See Table 1.
century on the fluorides and oxides [14], which are not
isostructural all along the series. Thus, the observed
trend of the Ln–ligand distance vs. the f n electronic
configuration of the lanthanide ion for isostructural
series of molecular compounds is of special signifi-
cance. This is done, in our paper, for both the Ln–O
and the Ln–Cl bond distances, i.e. for five different sets
of data.
(yellow), TmCl3(H2O)6 (colourless), YbCl3(H2O)6 (col-
ourless) were obtained in substantially quantitative
yields. Elemental analyses were performed for chlorine
by the Volhard method and for the lanthanide content by
calcining the samples at 850 °C, after treatment with
some drops of concentrated HNO3, and finally by
weighing the residue as Ln2O3, or as CeO2, Pr6O11,
Tb4O7 [15a]. The IR spectra were measured with a FT-
IR Perkin–Elmer mod.1725X spectrophotometer.
2. Experimental
2.1. Synthesis of the DME adducts LnCl3 (Ln ¼ La, Nd)
from Ln2O3
Solvents were freshly distilled over the conventional
drying agents under a controlled atmosphere (dinitrogen
or argon). The reactions were carried out under an initial
atmosphere of dinitrogen or argon. In this connection, it
is important to realize that since gases (HCl, SO2, CO2)
are produced during the preparation of the anhydrous
lanthanide chlorides, the initial anhydrous conditions
can be maintained simply by using a no-return valve
filled with an inert liquid (usually Nujol) in order to keep
the content of the reaction flask separated from the ex-
ternal atmosphere. Some of the lanthanide oxides
The preparations of the europium and gadolinium
complexes LnCl3(DME)2, Ln ¼ Eu, Gd, from the cor-
responding oxides were reported in our preliminary
communication [5] and will not be repeated here. The
preparation of the neodymium derivative, already an-
nounced earlier [5], is reported here for some important
details concerning the crystallization of the DME ad-
duct. Lanthanum. The oxide La2O3 (5.00 g, 15.3 mmol)
was suspended in DME (200 cm3) and thionyl chloride
was added (50 cm3, d ¼ 1:631 g ꢃ cmꢁ3, 685 mmol).
Water (8 cm3, 444 mmol) was then added at room
temperature in about 2.5 h. After 3 days stirring the
suspension was filtered under dinitrogen and the col-
ourless solid was dried in vacuo (10ꢁ2 Torr, ca. 4 h).
The product, analysing as LaCl3(DME), was sealed in
vials under dinitrogen (9.67 g, 94% yield). Anal. Calc.
for C4H10Cl3LaO2: Cl, 31.7; La, 41.4. Found: Cl, 31.9;
La, 41.4%. IR (1250–850 cmꢁ1, Nujol mull, cmꢁ1):
1234m, 1186m, 1144w, 1082s, 1039s, 1001m, 857s. From
the filtrate, stored at 0 °C, colourless crystals separated
^
(Nd2O3, Sm2O3, Gd2O3) were supplied by Rhone-Pou-
lenc. Commercial (Aldrich, C. Erba, Fluka, Strem) hy-
drated cerium, praseodymium, erbium and lutetium
trichlorides were used. The hydrated trichlorides of ter-
bium, holmium, thulium and ytterbium were prepared
by heating the metal oxide (Ln2O3, with the exception of
terbium for which Tb4O7 [15a] was used) with concen-
trated aqueous HCl. The resulting solution was then
evaporated to dryness in vacuo at ca. 50 °C. Residues
analysing as TbCl3(H2O)5 (colourless), HoCl3(H2O)5:5

U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
1541
out, which were chosen, before being dried, for the X-
2.3. The LnCl3(DME)2 derivatives
ray diffractometric studies and found to contain the
dinuclear molecules [LaCl3(DME)2]2 (vide infra). Neo-
dymium. The oxide Nd2O3 (4.56 g, 13.5 mmol) was
suspended in DME (100 cm3) and thionyl chloride (30
cm3, 411 mmol) and water (4 cm3, 222 mmol) were
added at room temperature over about 2 h. After 1 day
stirring, the suspension was filtered under dinitrogen
and the violet solid was dried in vacuo (10ꢁ2 Torr, ca. 6
h). The product was collected under dinitrogen and
shown to have the analytical composition NdCl3(DME)
(7.99 g, 87% yield). Anal. Calc. for C4H10Cl3NdO2: Cl,
31.2; Nd, 42.3. Found: Cl, 32.3; Nd, 43.2%. IR (1250–
850 cmꢁ1, Nujol mull, cmꢁ1): 1243m, 1162w, 1117s,
1049s, 1027s, 863s. Violet crystals of Nd2Cl6(DME)4
separated out from the filtrate at 0 °C: they were iso-
lated by filtration and used for the X-ray diffractometric
studies.
The samarium derivative was also obtained by a
similar procedure starting from Sm2O3. The pale yellow
DME adduct of SmCl3, analysing correctly as
SmCl3(DME)2, was also synthesized and used as an
intermediate to the corresponding N,N-di-iso-propyl-
carbamato derivative [15b].
The oxides of terbium and ytterbium, Tb4O7 and
Yb2O3 were unaffected by the treatment with SOCl2/
H2O/DME, the metal oxide being recovered unchanged.
By using the same procedure described for cerium,
products analysing as LnCl3(DME)2 were obtained.
Terbium. 82% yield. Anal. Calc. for C8H20Cl3O4Tb: Cl,
23.9; Tb, 35.7. Found: Cl, 23.5; Tb, 36.4%. IR (1250–850
cmꢁ1, Nujol mull, cmꢁ1): 1250m, 1241m, 1209w, 1153w,
1096vs, 1077s, 1030s, 977w, 859s. Colourless crystals of
TbCl3(DME)2 were obtained from the filtrate at )30 °C.
Holmium. 80% yield. Anal. Calc. for C8H20Cl3HoO4: Cl,
23.6; Ho, 36.5. Found: Cl, 23.5; Ho, 37.1%. IR (1250–850
cmꢁ1, Nujol mull, cmꢁ1): 1250m, 1241m, 1209w, 1154w,
1095vs, 1076s, 1031s, 976s, 857s. Pale yellow crystals of
HoCl3(DME)2 were obtained from the filtrate at )30 °C.
Erbium: 90% yield. Anal. Calc. for C8H20Cl3ErO4: Cl,
23.4; Er, 36.9. Found: Cl, 22.4; Er, 37.2%. Thulium. 85%
yield. Anal. Calc. for C8H20Cl3O4Tm: Cl, 23.4; Tm, 37.1.
Found: Cl, 23.1; Tm, 37.6%. IR (1250–850 cmꢁ1, Nujol
mull, cmꢁ1): 1250m, 1241m, 1209w, 1153w, 1094m,
1076s, 1032s, 976s, 856s. Colourless crystals of
TmCl3(DME)2 were obtained from the filtrate at about 0
°C. Ytterbium: 87% yield. Anal. Calc. for C8H20Cl3O4Yb:
Cl, 23.1; Yb, 37.6. Found: Cl, 22.8; Yb, 37.2%. IR (1250–
850 cmꢁ1, KBr pellets, cmꢁ1): 1249m, 1242m, 1209m,
1186m, 1153w, 1117m, 1091s, 1077s, 1032vs, 977m, 861s.
Colourless crystals of YbCl3(DME)2 were obtained from
the filtrate at about 0 °C. Lutetium. 90% yield. Anal. Calc.
for C8H20Cl3LuO4: Cl, 23.0; Lu, 37.9. Found: Cl, 22.6;
Lu, 38.1%. IR (1250–850 cmꢁ1, Nujol mull, cmꢁ1):
1240w, 1185w, 1119m, 1076m, 1029s, 978w. Colourless
crystals of LuCl3(DME)2 were obtained from the filtrate
at about 0 °C.
2.2. Synthesis of the DME adducts of LnCl3 (Ln ¼ Ce,
Pr, Tb, Ho, Er, Tm, Yb, Lu) from the hydrated chloride
The syntheses were carried out in DME starting from
LnCl3(H2O)x by using SOCl2 as dehydrating agent.
Cerium. The commercial product CeCl3(H2O)7 (9.78 g,
24.9 mmol) was suspended in DME (100 cm3) and
thionyl chloride was added (25 cm3, 343 mmol) from a
dropping funnel in 2 h. After 1 day stirring the sus-
pension was filtered under dinitrogen and the colourless
solid was dried in vacuo (10ꢁ2 Torr, ca. 5 h). The re-
sulting product, analysing as CeCl3(DME), was sealed
in vials under dinitrogen (8.29 g, 99% yield). Anal. Calc.
for C4H10CeCl3O2: Cl, 31.6; Ce, 41.6. Found: Cl, 31.3;
Ce, 41.8%. IR (1250–850 cmꢁ1, Nujol mull, cmꢁ1):
1233m, 1107m, 1082s, 1039s, 859s. The product was
recrystallized from DME: at 0 °C colourless crystals of
Ce2Cl6(DME)4 separated out and were used for the
X-ray diffractometric experiment, see Table 2. Praseo-
dymium. By using the same procedure described for
cerium, starting from PrCl3(H2O)6, the light yellow–
green product, analyzing as PrCl3(DME), was obtained
(78% yield). Anal. Calc. for C4H10Cl3O2Pr: Cl, 31.5; Pr,
2.4. Synthesis of ErCl3(DME)2 from Er2(CO3)3
The carbonate Er2(CO3)3(H2O)2 (5.56 g, 20.2 mmol
of Er) was suspended in DME (100 cm3) and thionyl
chloride was added (40 cm3, 548 mmol). Water (4 cm3,
222 mmol) was then added at room temperature in
about 0.5 h, and, after 3 d stirring, the suspension was
filtered under argon and the pink solid was dried in
vacuo at room temperature (10ꢁ2 Torr, ca. 6 h). The
product analysed as ErCl3(DME)2 (7.40 g, 81% yield).
Anal. Calc. for C8H20Cl3ErO4: Cl, 23.4; La, 36.9.
Found: Cl, 22.7; Er, 37.4%. IR (1250–850 cmꢁ1, KBr
pellets, cmꢁ1): 1241m, 1209m, 1186m, 1090s, 1076s,
1032vs, 976m, 858s. The filtrate, stored at 0 °C, sepa-
rated out pink crystals of the product. Similar results
were obtained starting from neodymium carbonate.
2.5. Reaction of Nd2O3 with HCl in DME: formation of
NdCl3(H2O)(DME)
41.8. Found: Cl, 31.9; Pr, 41.6%. IR (1250–850 cmꢁ1
,
Nujol mull, cmꢁ1): 1250w, 1234m, 1185w, 1144w,
1108m, 1082s, 1039s, 997w, 859s. The filtrate, stored at
about 0 °C, separated out pale yellow crystals of
Pr2Cl6(DME)4.
Under a N2 atmosphere, the oxide Nd2O3 (29 mmol of
Nd) was suspended in DME (100 cm3) in a 500 cm3 flask.
The flask was then evacuated and connected to an

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U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
apparatus producing HCl from NH4ClðsÞ and concen-
trated H2SO4. The suspension was stirred till gas ab-
sorption had ceased (17 h). The product was recovered
by filtration and dried in vacuo at room temperature for
7 h. (10.02 g, 96% yield). Anal. Calc. for NdCl3(H2O)-
(DME), C4H12Cl3NdO3: Cl, 29.6; Nd, 40.2. Found: Cl,
29.3; Nd, 40.2%. IR (PCTFE and Nujol, cmꢁ1): 3297s,
3168m, 2964w, 2945w,1636w, 1606m, 1559w, 1541w,
1508w, 1304w, 1279m, 1240w, 1192w, 1157w, 1107m,
1097m, 1048s, 1018m, 1011m, 864s, 829w.
The calculations were done by using the SHELX-97
and SHELXTL programmes [17,18]. In the final refine-
ment cycles based on F 2, anisotropic thermal parame-
ters were used for all non-hydrogen atoms. The final
reliability factors are listed in Tables 1 and 2.
3. Results and discussion
As partly announced in a preliminary communica-
tion, the oxides Ln2O3 of some early lanthanides
(Ln ¼ La, Nd, [5] Sm, Eu, [5] Gd [5]) are converted to
the corresponding chlorides [LnCl3(DME)2]n by treat-
ment with DME and thionyl chloride at room temper-
ature. The reaction can be represented as shown in the
general Eq. (1), which is the sum of the partial Eqs. (1a)–
(1c).
2.6. Treatment of EuCl3(H2O)2 with DME
The chloride EuCl3(H2O)2 (1.79 g, 6.08 mmol) was
introduced in a flask containing 100 cm3 of DME. The
suspension was stirred for 21 h at room temperature and
the colourless product was recovered by filtration and
dried in vacuo at room temperature for 5 h (2.04 g,
87.3% yield). Anal. Calc. for EuCl3(H2O)2(DME),
C4H14 Cl3EuO4: Cl, 27.7; Eu, 39.5. Found: Cl, 27.8; Eu,
38.6%. IR (Nujol, cmꢁ1) 3339s, 1633m, 1606m, 1261w,
1250w, 1241w, 1188w, 1154w, 1104w, 1088w, 1078m,
1037s, 976w, 867s, 830w, 801w. A portion of the product
(1.81 g, 4.71 mmol) was further heated with DME at
the reflux temperature for 6 h. The solid, collected by
filtration, was dried in vacuo for 6 h at room tempera-
ture (1.30 g, 75.4% yield). Anal. Calc. for EuCl3(H2O)
(DME), C4H12Cl3 EuO3: Cl, 29.0; Eu, 41.5. Found: Cl,
29.1; Eu, 42.8%.
3H2O þ 3SOCl2 ! 3SO2 þ 6HCl
Ln2O3 þ 6HCl ! 3H2O þ 2LnCl3
2LnCl3 þ 2nDME ! 2LnCl3ðDMEÞn
ð1aÞ
ð1bÞ
ð1cÞ
Ln2O3 þ 3SOCl2 þ 2nDME
! 2LnCl3ðDMEÞn þ 3SO2:
ð1Þ
No reaction occurred between Ln2O3 and SOCl2
under similar conditions in the absence of DME/H2O,
in agreement with the observation that chlorination of
lanthanide oxides by thionyl chloride requires high
temperatures [19]. The reaction must of course be
carried out in the presence of a defective amount of
water with respect to SOCl2, and the attack of the
metal oxide is believed to be carried out by the hy-
drogen chloride produced in situ, see reaction (1a).
Although a catalytic amount of water is in principle
required, reasonable reaction rates are obtained if en-
ough water is present to nominally produce a two-
molar excess of HCl with respect to the stoichiometry
of Eq. (1b). A twofold molar excess of thionyl chloride
with respect to water is employed in order to maintain
the system under final anhydrous conditions. In this
connection, it is remarkable that the presence of thio-
nyl chloride is essential for the formation of the an-
hydrous etherate of LnCl3: as a matter of fact, in the
case of neodymium, it has been shown that the oxide
Nd2O3, treated with dry HCl in DME produces the
aquo complex of analytical composition NdCl3(H2O)
(DME). Similar general behavior is expected for other
lanthanide oxides.
2.7. X-ray crystallography
Only the structure determination of TbCl3(DME)2 is
described, the other determinations being performed in
a similar way, see Tables 1 and 2.
The diffractometric measurements were carried out
by using a Bruker P4 diffractometer equipped with
graphite-monochromated MoKa radiation (k ¼ 0:71073
ꢂ
A). One of the colourless crystals was sealed in a glass
capillary under a dinitrogen atmosphere and the unit
cell parameters listed in Table 1 were derived from the
setting angles of 31 strong reflections. Data were col-
lected in the x=2h scan mode, and three standard re-
flections were monitored every 97 measurements for
checking crystal decay and equipment stability. Data
reduction was carried out by the XSCANS programme
[16]. A set of 3621 intensity data were collected between
h ¼ 2:6° and 25.0° and corrected for Lorentz and po-
larization effects and for absorption by the w-scan
method. After merging the equivalent reflections
ꢂ
ꢀ
P
ꢁ
ꢃ
P
Rint
¼
jFo2 ꢁ Fo2ðmeanÞj= ðFo2Þ ¼ 0:0315 , 2746
unique reflections were retained for the following cal-
culations. The systematic absences indicated the P21=c
space group. The solution was found by direct methods
and the molecule was completed in the subsequent dif-
ference Fourier maps.
It is to be noted that due to the higher coordinating
power of DME with respect to thionyl chloride [20], no
case was ever observed of SOCl2 being coordinated to
the final product. The rather strong coordinating power
of DME is also considered to be an important factor in

U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
1543
the successful formation of the anhydrous chlorides,
since coordination by DME presumably leads to more
soluble species of lower nuclearity. It has in fact been
found that addition of DME to EuCl3(H2O)2 produces
the DME adduct EuCl3(H2O)2(DME), or EuCl3(H2O)
(DME), depending on temperature. Finally, it is to be
observed that although in principle this methodology
operates also with tetrahydrofuran (THF) as reported
earlier [5], the use of DME is preferred because THF
may dehydrate, as some of us have observed under
certain conditions [21].
systematically present in the second part (from samar-
ium to lutetium), as it will be outlined below. Neo-
dymium appears to be the turning point between the two
types of compounds, since it has been isolated both as
the dinuclear species, vide infra, and as the mononuclear
derivative [12].
We shall describe the mononuclear complexes first.
4. Mononuclear derivatives
The SOCl2/H2O/DME reaction works well for the
synthesis of the LnCl3(DME)n adducts of the lighter
lanthanides. For the heavier elements, we have found it
more convenient to prepare the Tb, Ho, Er, Tm, Yb and
Lu compounds by dehydrating the easily accessible
lanthanide chlorides with SOCl2 in DME, see Eq. (2).
The hydrated starting materials LnCl3(H2O)n are readily
obtained from the oxides with aqueous hydrochloric
acid. After evaporation to dryness at a relatively low
temperature in order to avoid partial hydrolysis of the
metal chloride to the corresponding oxychloride, water
is completely removed by treatment with SOCl2 in
DME. Also the cerium and praseodymium derivatives
have been prepared according to Eq. (2), with complete
consumption of coordinated water.
The mononuclear adducts LnCl3(DME)2 from neo-
dymium to lutetium, see Table 1, crystallize in prismatic
shapes with the typical habit of Fig. 1. These isotypical
compounds have the molecular structure shown in
Fig. 2.
The metal is heptacoordinated with a distorted
pentagonal bipyramidal geometry where the Cl(1) and
Cl(3) atoms occupy the axial positions. The choride li-
gands form a plane approximately containing the metal
and separating the two ethereal ligands on opposite
sides.
Within each lanthanide compound the three Ln–Cl
distances show little variation, and in some cases are
practically identical, see Table 3.
On the contrary, the four Ln–O distances within each
lanthanide derivative are significantly different and one
short distance [Ln–O(3)], two intermediate distances
[Ln–O(1) and Ln–O(2)] and a long one [Ln–O(4)] are
present, see Table 4.
LnCl3ðH2OÞn þ nSOCl2 þ mDME
! LnCl3ðDMEÞm þ nSO2 þ 2nHCl:
ð2Þ
It is interesting to note that the DME adducts of the
lanthanide chlorides can also be prepared starting from
the appropriate metal carbonate by treatment with
SOCl2/H2O/DME, as exemplified by the erbium and
neodymium derivatives. Since neodymium and erbium
are separated by eight atomic numbers along the lan-
thanide series, it is anticipated that this route to the
anhydrous DME-solvated chlorides might be successful
at least with the other carbonates in between.
The isotypical series is large enough to show a sig-
nificant trend of metal–ligand distances vs. the number
For preparative purposes, the metal complexes
[LnCl3(DME)2]n, n ¼ 1 or 2, have been separated by fil-
tration and dried in vacuo at room temperature. How-
ever, it should be noted that the derivatives of the early
lanthanide elements, namely those in the sequence from
lanthanum to neodymium tend to lose DME in vacuo at
room temperature giving compounds of analytical
composition LnCl3(DME), presumably polynuclear.
All these compounds show almost superimposable IR
spectra with typical bands at 1030–1036, 1076–1079 and
1091–1096 cmꢁ1, attributable to coordinated DME.
Extensive use of X-ray diffractometry has been neces-
sary to rationalize the molecular complexity of the
products. It has therefore now been established that
dinuclear compounds are formed preferentially in the
first part of the lanthanide series (from lanthanum to
neodymium), whereas mononuclear compounds are
Fig. 1. Crystal habit of the mononuclear complexes LnCl3(DME)2.
Fig. 2. The molecular structure of the mononuclear lanthanide deriv-
atives LnCl3(DME)2. Thermal ellipsoids are at 30% probability.

1544
U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
Table 3
Ln–Cl distances (A) in isotypical LnCl3(DME)2 structures
ꢂ
Element
Ln–Cl(1)
Ln–Cl(2)
Ln–Cl(3)
Ln–Cl mean
Nd [12]
Sm [15b]
Eu
2.681(1)
2.656(1)
2.643(1)
2.629(2)
2.615(1)
2.596(1)
2.592(2)
2.585(2)
2.577(3)
2.558(1)
2.557(3)
2.673(1)
2.647(1)
2.639(1)
2.625(2)
2.610(1)
2.598(1)
2.593(2)
2.582(2)
2.569(3)
2.563(1)
2.551(3)
2.682(1)
2.656(1)
2.643(1)
2.632(2)
2.617(1)
2.599(1)
2.590(2)
2.580(2)
2.573(3)
2.558(1)
2.548(3)
2.679(1)
2.653(1)
2.639(1)
2.629(1)
2.614(1)
2.598(1)
2.591(1)
2.586(1)
2.573(2)
2.559(1)
2.552(2)
Gd [10]
Tb
Dy [9a]
Ho
Er [7b,9b]
Tm
Yb
Lu
Table 4
ꢂ
Ln–O distances (A) in isotypical mononuclear LnCl3(DME)2 com-
plexesa
Fig. 3. Fitting of Ln–Cl and Ln–O distances for the mononuclear
LnCl3(DME)2 complexes vs. the number of f electrons.
Element
Ln–O(1)
Ln–O(2)
Ln–O(3)
Ln–O(4)
Nd [12]
Sm [15b]
Eu
2.523(2)
2.504(3)
2.481(3)
2.463(4)
2.457(4)
2.441(3)
2.436(4)
2.427(4)
2.420(7)
2.413(2)
2.392(7)
2.525(2)
2.501(3)
2.487(3)
2.474(4)
2.464(4)
2.450(3)
2.441(5)
2.423(4)
2.414(6)
2.413(2)
2.390(9)
2.481(2)
2.448(3)
2.438(3)
2.413(4)
2.405(4)
2.396(3)
2.389(5)
2.370(4)
2.368(7)
2.357(2)
2.349(8)
2.560(2)
2.534(3)
2.513(3)
2.494(4)
2.500(4)
2.472(3)
2.478(4)
2.462(4)
2.456(7)
2.449(2)
2.421(8)
Table 5
Contraction of Ln–X distances: the A parameters resulting from the
fitting of Fig. 3, and the correlation factor R2
Gd [10]
Tb
Dy [9a]
Ho
Bond
A0
A1
A2
R2
Ln-Cl
2.724(5)
2.569(9)
2.568(9)
2.534(9)
2.600(17)
0.016(1)
0.017(2)
0.014(2)
0.020(2)
0.015(4)
0.00023(7)
0.0003(1)
0.0001(1)
0.0005(1)
0.0002(2)
0.997
0.989
0.991
0.991
0.965
Er [7b,9b]
Tm
Yb
Ln–O(1)
Ln–O(2)
Ln–O(3)
Ln–O(4)
Lu
a For atom numbering see Fig. 2.
of f electrons of the metal ion, and this can be done for
the four types of Ln–O distances and for the Ln–Cl
distances in the sequence from neodymium to lute-
tium. 3 As there are no significant differences in the Ln–
Cl distances within each lanthanide, see Table 3, the
mean values with the appropriate standard deviations
have been considered. The plots of Ln–Cl and Ln–O
distances vs. the number of f electrons of Ln3þ are
shown in Fig. 3.
A quadratic fit of the plotted values with the function
[23] d(Ln–Cl) ¼ A0 ꢁ A1n þ A2n2, gave the values listed
in Table 5 for the A0, A1 and A2 coefficients.
As it can be seen from the values of the coefficients
used in the fitting of Ln–Cl and Ln–O(3) distances, A2 is
significantly different from zero.
It is finally worth mentioning that in these isotypical
structures the unit cell volume is related to the nature of
the central lanthanide. Fig. 4 shows a plot of these data.
The cell volumes which have been measured at room
temperature well fit the function U ¼ 1614ð3Þ ꢁ 7:7ð9Þf
Fig. 4. Fitting of cell volumes of LnCl3(DME)2 measured at room
temperature vs. the number of f electrons of the lanthanide. The values
referring to the Eu, see Table 1, and Dy [9a] compounds have been
omitted from the fitting because data collection for these compounds
was carried out at 208 and 203 K, respectively. As far as the Gd de-
rivative is concerned, although the CCDC indicates data collection at
room temperature, the considerably lower value of the cell volume
3
The means and the standard deviations from the means have
ð1=r2i Þ;
ꢄ
P
P
3
been calculated [22] by the expressions: ꢃx ¼ ðxi=r2i Þ
ꢂ
[1530 A ] reported for this compound suggests that the experiment was
actually carried out at a lower temperature.
ꢀ
ꢁ
P
1=2
rðꢃx ¼ 1= ð1=r2i Þ
.

U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
1545
þ0:11ð5Þf 2, with a correlation factor R2 ¼ 0:996. The
cell volumes of the Eu and Dy complexes have been
omitted from the fitting because they have been mea-
sured at low temperature.
5. Dinuclear derivatives
As the metal radius increases, heptacoordination be-
comes progressively insufficient to envelop completely
the metal centre: with neodymium, octacoordination
appears, as shown by a recent X-ray diffractometric
experiment on Nd2Cl6(DME)4 [9c]. As the atomic
number of the lanthanide decreases, the mononuclear
species LnCl3(DME)2, become progressively unstable.
The neodymium complex may be considered as the
borderline case, for which both the mononuclear [12]
and the dinuclear, chloride-bridged dinuclear species
Nd2Cl4(l2-Cl)2(DME)4 [9c] have been identified in the
solid state.
As shown by the results of this paper, only dinuclear
complexes were observed for lanthanum, cerium and
praseodymium. At odds over the mononuclear ones,
for which only one crystal phase has been obtained
until now, the dinuclear species show an extended
polymorphism.
A monoclinic phase with the P21=n space group and
with the cell parameters listed in the first four columns
of Table 2 has been obtained for the La, Ce, Pr and Nd
complexes (the products were recrystallized from DME
in the temperature gradient from room temperature to
about 0 °C). Although the structure of Nd2Cl4(l2-
Cl)2(DME)4 [9c] had already been reported, we decided
to investigate the structure of the neodymium derivative,
as obtained under our conditions. It constitutes a new
phase of Nd2Cl4(l2-Cl)2(DME)4, that we propose to call
a, whose crystals have the habit shown in Fig. 5. They
are isotypical with those of La2Br4(l2-Br)2(DME)4 [7b].
The molecular structure of the a-phase is shown in
Fig. 6.
Fig. 6. The molecular structure of a-Ln2Cl6(DME)4, Ln ¼ La, Ce, Pr
and Nd. Thermal ellipsoids are at 30% probability.
Tables 6 and 7 list the Ln–Cl and Ln–O bond dis-
tances for the a-phase of the La, Ce, Pr and Nd dinu-
clear compounds. For the specific case of neodymium,
an interesting comparison can be made about bond
distances. The average Nd–Cl bond distance for the
ꢂ
terminal chloride ligands in our dimer is 2.71 A, to be
ꢂ
compared, see Table 3, with the 2.68 A average distance
of the Nd–Cl bond in the mononuclear compound.
Moreover, by comparing the data of Table 4 with those
of Table 7, it can be concluded that the Nd–O bond
distances in the neodymium dimer are all systematically
ꢂ
less than 0.1 A longer than in the corresponding
mononuclear compound.
The Ln–O bond distances of the dinuclear com-
pounds show significantly different values: two are in-
termediate, one short and one long.
As expected for the increase of the coordination
number, the dinuclear complexes are more densely
packed than the mononuclear ones. If we compare, in
fact, the volume occupied by one unit of NdCl3(DME)2
Table 6
The molecule is centrosymmetric and the coordina-
tion polyhedron may be considered as intermediate be-
tween the eight-vertex dodecahedron and the bicapped
trigonal bipyramid. The four chlorine atoms are ar-
ranged around the metal in a sort of flattened tetrahe-
dron, the dinuclear molecule resulting from the sharing
of the Cl(1)–Cl(10) edge. The DME oxygens around the
metal centre form another flattened tetrahedron.
ꢂ
Ln–Cl distances (A) in isotypical a-Ln2Cl6(DME)4
Element
Ln–Cl(1)
Ln–Cl(10)
Ln–Cl(2)
Ln–Cl(3)
La
Ce
Pr
2.902(2)
2.884(1)
2.863(1)
2.849(2)
2.949(2)
2.927(1)
2.912(1)
2.900(1)
2.745(3)
2.728(1)
2.709(1)
2.693(2)
2.791(2)
2.770(1)
2.751(1)
2.742(2)
Nd
Table 7
ꢂ
Ln–O distances (A) in isotypical a-Ln2Cl6(DME)4 structures
Element
Ln–O(1)
Ln–O(2)
Ln–O(3)
Ln–O(4)
La
Ce
Pr
2.623(6)
2.608(3)
2.587(2)
2.575(4)
2.619(6)
2.600(3)
2.586(3)
2.573(4)
2.592(6)
2.584(3)
2.574(2)
2.556(4)
2.674(7)
2.666(4)
2.654(3)
2.637(4)
Nd
Fig. 5. Crystal habit of Nd2Cl6(DME)4.

1546
U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
3
ꢂ
in the mononuclear case (V =Z ¼ 397:9 A ) with the
volume occupied by the same unit in the dinuclear
0
0
3
ꢂ
compound [V =ð2 ꢀ Z Þ ¼ 383:5 A ], we observe a de-
crease of 3.6%.
By recrystallizing the a-phase of Pr2Cl6(DME)4 from
DME, we found prismatic crystals with a more elon-
gated shape, in addition to the crystals with the habit of
Fig. 5. One of them showed the same monoclinic space
group P21=n as the a-phase, but a different unit cell
ꢂ
(a ¼ 8:474, b ¼ 18:149, c ¼ 10:569 A; b ¼ 109:16°), see
Table 2. Its crystal structure solution showed to be a
polymorph of the a-phase, built by molecules with
the same conformation, but with a different packing.
Fig. 7 shows a comparison between the crystal struc-
tures of the a-phase and the new one that we propose
to call b.
Fig. 8. The molecular structure of Nd2Cl6(DME)4 ꢀ DME. Thermal
ellipsoids are at 30% probability.
The more evident difference between the two struc-
tures can be found in the disposition of the {Pr2Cl2}
lozenges forming the central units of the molecules. In
the structure of the a-phase they are lying on parallel
planes, while in the b one the lozenges of adjacent rows
are disposed on almost perpendicular planes. This cor-
responds to a slightly more efficient packing and, con-
sequently, to a higher density.
The coordination geometry around the metal is ap-
proximately square antiprismatic. It is almost, but not
exactly, superimposable to the conformation observed
in the recently described structure of unsolvated
Nd2Cl6(DME)4 [9c], that we propose to identify as the
c-phase.
The Ln–Cl and Ln–O distances of Nd2Cl6(DME)4 ꢀ
DME are listed in Table 8. The interatomic distances are
very similar to those of the c-phase. No bonding inter-
actions with the uncoordinated DME molecules are
present in this neodymium complex. As displayed in
Fig. 9, the additional DME molecules forming the crystal
constitute a spacer layer between adjacent dinuclear
molecules.
At the present time, only the a-phase of the dinuclear
complexes of lanthanum(III), and cerium(III) have been
found. In our recrystallizations of the neodymium de-
rivatives, we have identified crystals of another type,
shaped as little sticks. These crystals correspond to a
new solvated phase of Nd2Cl6(DME)4 ꢀ DME. They
ꢃ
belong to the triclinic space group P1, with cell param-
ꢂ
eters: a ¼ 8:645, b ¼ 9:322, c ¼ 12:676 A; a ¼ 88:60,
b ¼ 83.88, c ¼ 65.78°. The molecular structure of the
dinuclear compound is shown in Fig. 8.
Table 8
ꢂ
Bond distances (A) in Nd2Cl6(DME)4 ꢀ DME
Also this dinuclear complex is centrosymmetric, but
performs a different conformation with respect to that of
the a-phase. As it can be seen, the Cl(1) Cl(10) Cl(2) and
Cl(3) atoms around neodymium form a distorted tri-
gonal pyramid, with the metal lying on the basal plane.
Ln–Cl(1)
Ln–Cl(10)
Ln–Cl(2)
Ln–Cl(3)
2.828(1)
2.958(1)
2.661(1)
2.708(1)
Ln–O(1)
Ln–O(2)
Ln–O(3)
Ln–O(4)
2.592(4)
2.503(4)
2.604(4)
2.631(4)
Fig. 7. The different crystal structures of a and b-Pr2Cl6(DME)4. The
molecules are projected down c and down a, respectively.
Fig. 9. Crystal structure of Nd2Cl6(DME)4 ꢀ DME projected down b.

U. Baisch et al. / Inorganica Chimica Acta 357 (2004) 1538–1548
1547
6. Conclusions
bridge Crystallographic Data Centre with the reference
numbers which follow. For the LnCl3(DME)2 series
(Ln ¼ Eu, Tb, Ho, Tm, Yb, Lu): CCDC 211399, CCDC
211400, CCDC 211401, CCDC 211402, CCDC 211403,
CCDC 211404, respectively; for the a-Ln2Cl6(DME)4
series (Ln ¼ La, Ce, Pr, Nd): CCDC 211405, CCDC
211406, CCDC 211407, CCDC 211408, respectively; for
Nd2Cl6(DME)4 ꢀ DME: CCDC 211409; and for b-
Pr2Cl6(DME)4: CCDC 211410. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK, (fax: +44 1223
336033 or deposit@ccdc.cam.ac.uk).
Three different methods for the preparation of the
DME adducts of lanthanide trichlorides are presented
in this paper, starting from the readily available oxide,
from the hydrated chlorides or from the carbonates, thus
substantially covering the whole series of lanthanide ele-
ments. The methods operate at room temperature, and as
long as moisture is avoided, no special care should be
taken to operate under an inert atmosphere. The failure of
the oxides of terbium and ytterbium, i.e. towards the end
of the series, to undergo chlorination by the SOCl2/DME/
H2O system is believed to be thermodynamic in nature
and related to the elevated lattice energy of the oxide.
The new structural information from this paper shows
that the lighter lanthanides, characterized by a relatively
large ionic radius, form isotypical dinuclear Ln2
Cl6(DME)4 complexes. Although cases of polymorphism
have been identified, in all cases the central metal atom is
octacoordinated in the dinuclear species. For neodym-
ium, in addition to the already known [12] mononuclear
species NdCl3(DME)2, also the dinuclear Nd2Cl6(DME)4
has been identified in the framework of the present study.
Thus, neodymium, with its relatively large ionic radius,
appears to be the borderline element for which heptaco-
ordination in the mononuclear NdCl3(DME)2 derivative
and octacoordination in Nd2Cl6(DME)4 are character-
ized by a small energy difference and substantially even
coexist in the solid state.
Acknowledgements
The authors wish to thank the Ministero dellÕ Ist-
ꢀ
ruzione, dellÕ Universitae della Ricerca (MIUR), Pro-
grammi di Ricerca Scientifica di Rilevante Interesse
Nazionale, Cofinanziamento 2002–2003, the Consiglio
Nazionale delle Ricerche (CNR), Materiali Speciali per
Tecnologie Avanzate II (MSTA II), Roma, for financial
^
support of this work, and Rhone-Poulenc for a generous
gift of lanthanide oxides. Thanks are due to Dr. Ales-
sandra Merigo for some preliminary synthetic experi-
ments.
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